The invention relates to a freight container for flowable materials, which comprises a tank with circumferentially extending reinforcing rings, end frames, and diagonal members interconnecting corner fittings of the end frames with the reinforcing rings.
According to ISO standard 1496/3, the structure of a tank container must be such that the lower corner fittings thereof alone are able to accommodate all live loads. As from a specified length containers have to be provided, in addition to said corner fittings, with at least two further load receiving zones at an axial distance between 1.7 and 2.0 m from the end frames in order to ensure an improved introduction of forces into the frame side members of the road vehicle.
In the case of frame-type tanks these bearing zones may be provided by the insertion of correspondingly rigid frame cross members in the standard region between the lower frame side members of the container. It is then possible to mount on these cross members the longitudinal or transverse saddle members through which the tank is bolted or welded to a body platform.
Moreover, an ISO container has to be dimensioned so as to be able to accommodate tensile loads and forces of pressure in longitudinal direction of the body platform of up to twice the total gross weight of the container.
In the case of frame-type tanks, these longitudinal forces occurring in operation and thus the bending forces acting on the end frames are accommodated by the frame side members of the body platform. However, they may also be accommodated by diagonal members extending approximately in the direction of the centroid of the body platform, said diagonal members introducing said forces either into a bottom beam and thus beneath the tank body or introducing them into the tank casing via suitable transition pieces.
Such a freight container, which is known from U.S. Patent Specification No. 4,307,812, is provided with face-side rectangular or square frames which are used to accommodate the hoisting and stacking forces and are joined to each other through a body platform for accommodating the axial forces. The body platform consists of one or two frame side members and of diagonal members joined to the end frames. As an advantageous embodiment there is also described therein a coupling between these diagonal members and the saddle mount of the tank container. To this end the point of intersection between the members extending diagonally towards the centre of the tank and a reinforcing ring of the cylindrical tank lies within the lower bearing zones prescribed according to ISO specifications (ISO-1496/3).
Self-supporting or "beam tanks", as they are called, in which the tank body itself accommodates the longitudinal forces and wherein no body platform is provided, present difficulties both in respect of the provided ISO bearing zones and in respect of the diversion of the longitudinal forces. Provided the forces are not accommodated by longitudinal members, the end groups of the beam tanks are subjected to considerable bending loads.
In order to provide the bearing zones for beam tanks it has been common practice to attach by welding more or less heavy saddle structures, which actually are not necessary for transporting the tank container. Consequently, there result drawbacks in respect of the dead weight and heat build-up zones in a possibly existing thermal insulation.
It is an object of the instant invention to provide also for such self-supporting tank containers a statically safe solution, which is weight-saving and easy to manufacture and satisfies the demands both in respect of the ISO bearing zones and in respect of the diversion of the longitudinal forces equally well. The joint between the tank body and the container frame shall be fatigue-proof also during long-term operation and shall permit safe introduction of the vertical bearing forces and of the longitudinal inertial forces, while the dead weight is to be low and the close ISO tolerances between respective corner fittings are observed.